Solve optional data signing task

Lab 2 - Certificates/README.md

@@ -294,3 +294,32 @@ After that, signing works properly:
 ```sh
 openssl ca -config /root/ca/openssl.cnf -extensions v3_intermediate_ca -days 3650 -notext -md sha256 -in intermediate.req -out intermediate.cer
 ```
+
+Check the output:
+```sh
+openssl x509 -in intermediate.cer -noout -text
+```
+
+# Signing Data
+Create a file with secret content:
+```sh
+cd /root
+vi secret.txt
+```
+
+## Create Signature
+```sh
+# Create and encrypt hash of file
+openssl dgst -sha256 -sign /root/ca/private/ca.key.pem -out /tmp/hash.sha256 secret.txt
+# Convert hash file to base64
+openssl base64 -in /tmp/hash.sha256 -out secret.sig
+```
+
+## Verify Signature
+Under normal circumstances:
+```sh
+# Re-create hash file from base64
+openssl base64 -d -in secret.sig -out /tmp/hash.sha256
+# Check integrity of `secret.txt`
+openssl dgst -sha256 -verify /root/ca/ca.key.pem.pub -signature /tmp/hash.sha256 secret.txt
+```